Processes for preparing methylchlorosilanes by reaction of silicon with methyl chloride (direct synthesis) in the presence of suitable catalysts and catalyst combinations are already known. For example, U.S. Pat. RE No. 33,452 describes a direct synthesis process using a catalyst combination of the elements or the compounds of copper, zinc and tin. The ratio of the catalysts copper, zinc and tin to one another in the process has a strong influence on the process, in particular on the productivity and the selectivity, while the form, such as metal, alloys or compounds, in which the catalysts are introduced into the catalyst composition is of subsidiary importance. Furthermore, relatively low production rates are achieved using tin as catalyst.
D. I. Lainer et al., Chemical Abstracts 57, 6669g 1962, describe a direct synthesis process in which a silicon-copper alloy containing zinc and antimony as catalysts is used. If copper is not already alloyed in the preparation of the silicon, the preparation of silicon-copper alloys is energy intensive and requires additional plants. Since the ratio silicon: copper is predetermined by these alloys, the silane synthesis is more difficult to control. Changes in the Si:Cu ratio in the catalyst composition during the reaction cannot be quickly compensated for. Use of metallic copper gives, in comparison with copper oxides, lower silane production rates (silane per mass of silicon and unit time). However, silicon-copper oxide alloys are unstable and cannot be prepared because of the electrochemical series.
Besides copper-silicon alloys, copper is usually used in the form of metallic copper, copper chloride, copper formate or copper oxalate. The preparation of these copper compounds in relatively pure form and suitable particle sizes is energy and cost intensive. Recycling of used copper for reuse in the starting compound is relatively expensive. Copper formate and oxalate are thermally labile, i.e., they can decompose spontaneously on heating. Use of copper chloride results, because of the decomposition of methyl chloride, in increased tar and soot formation on the catalyst composition and thereby in a reduction in reactivity. Metallic copper has the disadvantage that it is ductile and therefore can be mechanically comminuted only with difficulty to give a powder having a suitable particle size and surface area. A further disadvantage of metallic copper catalysts is a lower selectivity in respect of dimethyldichlorosilane. For example, the use of copper oxalate in the direct synthesis together with elemental zinc and antimony is described in DD-A-293 506.
Lieske et al., Xth Intern. Symposium on Organosilicon Chemistry 1993, Poznan, Poland; Abstract page 209 describes a direct synthesis process in which a combination of copper oxide with zinc and antimony is used as catalyst. In the preparation of the catalyst composition, metallic zinc can spontaneously undergo an exothermic redox reaction with the added copper compounds, such as copper oxides, which endangers process safety. In the reaction of the catalyst composition with methyl chloride, metallic zinc is oxidized to ZnCl.sub.2. ZnCl.sub.2 is partly volatile at the temperatures at which the methylchlorosilane synthesis proceeds. ZnCl.sub.2 is therefore removed from the catalyst composition during the reaction and deposits on cold parts of the plant or is carried away with the silane stream. Likewise, if metallic zinc or ZnCl.sub.2 is used, the catalyst composition is greatly depleted in this component during the reaction with methyl chloride. To maintain the productivity of the process, it is necessary to meter in further amounts of this component; at the same time there results an enrichment of undesired amounts of zinc compounds in the silane, on parts of the plant and in reaction dusts.
U.S. Pat. No. 4,645,851 and U.S. Pat. No. 4,661,613 propose direct synthesis processes in the presence of a catalyst combination of copper oxide, zinc oxide, metallic antimony and an addition of a further alkali metal or alkaline earth metal or metal compound, such as cesium chloride, calcium chloride or barium carbonate, but the processes were only carried out using a catalyst combination of copper oxide, zinc oxide, metallic tin and an additive. The disadvantage of such additives is that they usually have to be used in relatively high concentrations to produce corresponding effects, that most of the additives are relatively expensive and after the reaction accordingly have to be recovered or landfilled. For example, to achieve a high dimethyldichlorosilane selectivity and to increase the productivity, a high cesium chloride content is required.